Issue #4: Added From trait (#7)

* #4 Added `From` trait

This adds the `from()` and `tryFrom()` methods to pure enums, and the `fromName()` and `tryFromName()` methods for all enum types.

* Code review changes

Dropped support for fragile indexes on pure enums

* Dropped unnecessary array_pop()

* import ValueError

* remove import

Co-authored-by: Samuel Štancl <samuel@archte.ch>
Co-authored-by: Samuel Štancl <samuel.stancl@gmail.com>

tests/Pest/FromTest.php

@@ -0,0 +1,65 @@
+<?php
+
+it('does not override the default BackedEnum from method')
+    ->expect(Status::from(0))
+    ->toBe(Status::PENDING);
+
+it('does not override the default BackedEnum from method with errors', function () {
+    Status::from(2);
+})->throws(ValueError::class, '2 is not a valid backing value for enum "Status"');
+
+it('does not override the default BackedEnum tryFrom method')
+    ->expect(Status::tryFrom(1))
+    ->toBe(Status::DONE);
+
+it('does not override the default BackedEnum tryFrom method with errors')
+    ->expect(Status::tryFrom(2))
+    ->toBe(null);
+
+it('can select a case by name with from() for pure enums')
+    ->expect(Role::from('ADMIN'))
+    ->toBe(Role::ADMIN);
+
+it('throws a value error when selecting a non-existent case with from() for pure enums', function () {
+    Role::from('NOBODY');
+})->throws(ValueError::class, '"NOBODY" is not a valid name for enum "Role"');
+
+it('can select a case by name with tryFrom() for pure enums')
+    ->expect(Role::tryFrom('GUEST'))
+    ->toBe(Role::GUEST);
+
+it('can returns null when selecting a non-existent case by name with tryFrom() for pure enums')
+    ->expect(Role::tryFrom('NOBODY'))
+    ->toBe(null);
+
+it('can select a case by name with fromName() for pure enums')
+    ->expect(Role::fromName('ADMIN'))
+    ->toBe(Role::ADMIN);
+
+it('throws a value error when selecting a non-existent case by name with fromName() for pure enums', function () {
+    Role::fromName('NOBODY');
+})->throws(ValueError::class, '"NOBODY" is not a valid name for enum "Role"');
+
+it('can select a case by name with tryFromName() for pure enums')
+    ->expect(Role::tryFromName('GUEST'))
+    ->toBe(Role::GUEST);
+
+it('returns null when selecting a non-existent case by name with tryFromName() for pure enums')
+    ->expect(Role::tryFromName('NOBODY'))
+    ->toBe(null);
+
+it('can select a case by name with fromName() for backed enums')
+    ->expect(Status::fromName('PENDING'))
+    ->toBe(Status::PENDING);
+
+it('throws a value error when selecting a non-existent case by name with fromName() for backed enums', function () {
+    Status::fromName('NOTHING');
+})->throws(ValueError::class, '"NOTHING" is not a valid name for enum "Status"');
+
+it('can select a case by name with tryFromName() for backed enums')
+    ->expect(Status::tryFromName('DONE'))
+    ->toBe(Status::DONE);
+
+it('returns null when selecting a non-existent case by name with tryFromName() for backed enums')
+    ->expect(Status::tryFromName('NOTHING'))
+    ->toBe(null);